Carbon Dioxide

Carbon Dioxide is a gas on earth, except in the form of dry ice, but it will
be a mineral in the outer solar system. It has the same symmetry as pyrite, and
virtually identical structure. In both cases, the cation has a face centered
cubic arrangement, but the anions are canted, reducing the symmetry. The
difference is that in pyrite the surrounding anions are all equally bonded to
the central iron, creating an octahedral coordination. In carbon dioxide, two
oxygen atoms flank a central carbon in a straight line. Since the molecules are
covalently bonded, it makes no sense to define coordination polyhedra, even if
the geometry is very similar to pyrite. Below is an oblique view of the carbon
dioxide unit cell. Oxygen atoms are in blue, with foreground atoms darker.

Carbon Dioxide, looking along a unit cell edge. Light colored molecules are
in the central plane of the cell. The top and bottom molecules have their tops
tilted into the diagram; the middle row has the bottom tilted inward.